The many magnitudes of increased sales and use which materialized once the electronic calculator was reduced to pocket size may be taken as an indication that pocket-sized information devices, in general, will find wide acceptance in the market place. A pocket-sized microform viewer, for example, would seem to constitute a highly desirable candidate for success in this context. Yet such a small viewer appears to be beyond the state of the art at the present time.
One apparent reason for the inability to realize a pocket viewer at this time has to do with blow-back. The term "blow-back" refers to the image size produced by, for example, a viewer with respect to original copy. Thus, a typical page of print 81/2 .times. 11 inches, when reduced to microfilm, would not be readable as an image projected on a pocket-sized viewer because such a viewer can have a viewing screen of only about 21/2 inches to 3 inches and this would allow a blow-back of an image of about one quarter the size of the original. Thus, it is clear that a pocket-sized microfilm reader cannot be realized presently because the dictated maximum screen size is insufficient to allow a legible image to be produced from source materials of 81/2 inches width or greater.
On the other hand, a non-real image device might be one possible route for realizing a viewer of such small dimensions. But further consideration indicates that this is not a possibility either. A virtual image device permitting viewing of the source material above is limited by the La Grange invariant. The term "La Grange invariant" characterizes the fact that all objects and images in a common optical system obey the relationship nh sin u = n'h' sin u' where n and n' are indices of refraction for the object and image space, sin u and sin u' are the angles of the marginal ray from an axial point in the object and image space, and h and h' are the heights of the object and image, respectively. In the simplest terms, the relationship means that for a 100x pocket viewer requiring an exit pupil of about a five inch diameter located at about 10 inches from a field lens (a relative aperture of f/2.0) would require an impossibly fast f/0.02 projection lens.
The non applicability of a virtual image approach in realizing a microfilm viewer may be appreciated when it is remembered that a microscope is a virtual image viewer. Such a viewer has so small an exit pupil, that for magnifications higher than about 12x it becomes strictly a one-eye viewing device with a rigidly fixed or optically defined eye position. Such a device is unacceptable to most users of microfilm. From a consideration of alternative optical system approaches it would appear that a pocket sized microfilm viewer is out of the question.
When the power supply for such a device is considered, the realization of the device seems even less likely. The term "pocket-sized" implies a self-contained power source. Real image devices with either front or rear projection screens, particularly the latter, are so inefficient that a battery-powered device of proper size would have a battery life of only a few minutes. Yet the necessity for a power cord is to be avoided. With a cord and with a standard screen with sufficient magnification (about 50x or greater) to use a sufficiently small microform to provide a useful storage capacity in a pocket device (100 pages or more), the film would be subjected to temperatures higher than the 129.degree. F. limit set by the National Micrographics Association because there is no room for a cooling fan. Thus, even from a power supply standpoint a pocket-size viewer is impractical.